1. Field of the Invention
This invention relates to a method for forming an emitter in a semiconductor device by diffusing a conductivity-determining impurity from a polysilicon "island" surrounded by a dielectric into an underlying, previously formed base region.
2. Prior Art
A commonly used process for forming an emitter in a semiconductor device is to provide polysilicon having an impurity of a conductivity-determining nature over a designated emitter site. (Solely for convenience of terminology, the portion of polysilicon overlying an emitter or base contact site is referred to as an "island". Depending on the shape of the site, the overlying polysilicon may be insular, peninsular, or otherwise shaped.) By controlled heating of the device, the impurities diffuse from the polysilicon into the underlying site to form a semiconductive region suitable for use as an emitter. Usually, the polysilicon over the emitter site will be left in place after the emitter is diffused to protect the emitter and also provide a suitable surface for use in forming an electrical connection to the emitter.
The usual method for defining polysilicon over an emitter site has been to form, in a series of steps, silicon dioxide or other suitably high-dielectric material over the portions of the wafer which will not be used for emitter sites. Polysilicon is then deposited over the exposed portions of the wafer and etched to remove it from undesired areas. After diffusion of impurities from the polysilicon overlying the emitter site into the wafer, suitable formation of ohmic contacts and metallization is then performed.
The problem with such a procedure is that the metallization step may cause a short between the contact to the base region and the polysilicon over the emitter site, preventing the proper functioning of the device. Such shorts are a result of the failure of the polysilicon etching step to remove all the polysilicon over the silicon dioxide around the emitter site polysilicon. Similarly, the subsequent metallization, given current etching techniques, cannot be precisely registered over the exposed portion of the base region to which electrical contact is desired. The metal being deposited over that exposed base region may itself be deposited over a portion of the silicon dioxide separating the emitter site from the base contact site. If the metal deposited over the base contact site is also deposited over that portion of the emitter polysilicon on the separating silicon dioxide, a direct short results.
Accordingly, it is an object of the herein disclosed invention to reduce the likelihood of an emitter-base short in a semiconductor device in which an emitter is formed from an overlaying polysilicon island.
It is a further object to more precisely determine the polysilicon island over an emitter site, thereby better separating the potential areas of shorting between an emitter and base.
Additionally, it is an object to provide for an emitter diffusion in which a conductivity-determining impurity may be incorporated into either a doped polysilicon layer or the impurity may be implanted into a previously deposited polysilicon layer, thereby permitting greater control of the electrical properties of an emitter.